Photovoltaic cells have been widely used in a variety of applications to generate convenient electricity. Typically, a single solar cell produces an output voltage around 0.5V, and a plurality of cells, typically Silicon based, is conventionally connected in series to provide higher voltage levels. Referring to FIG. 1a, multiple solar cell 22 are conventionally connected in series to form a “serial-unit” 26 of solar cells 22, wherein multiple serial-units 26 may be interconnected in series to form a string of serial-units 28, in order to obtain the desired output voltage in a solar-array module 20. Each serial-unit 26 may include one or more cells and is protected by a bypass diode 25 that is added to bypass local problems such as dirt, overcastting shades, other partial shading or otherwise malfunctioning cells.
Solar cells 22, being connected in series, suffer from the following setbacks:                a) Solar cells 22 may be subject to at least a partial light occlusion due to shading and/or dirt accumulated on one or more modules. Electric power generated in partially shaded cells is greatly reduced. An electric current produced by the cell is reduced proportional to the light intensity decreasing. Bypass diodes 25 enable the flow of electric current but does not compensate for the lost power from the bypassed serial-unit 26. Typically, the voltage drop on a diode 25 is about 0.25V.        b) Typically, solar array module 20 is sensitive to inverse breakdown voltage that may be developed in another solar-array module 20. Diode 27 prevents the breakage of the solar-array module. Diode 27 also prevents a solar-array module output short circuit.        c) Inequality between solar cells 22 also yields a loss in power.        
In an exemplary arrangement, for a nominal 30 volt Silicon solar-array module generating system, about 60 cells are connected in series to produce a 30 volt output. Usually, bypass diodes are placed across groups of cells, for example, 5-20 cells per diode instead of one bypass diode per cell to lower the cost. Cells connected in series with bypass diodes have been proven to be effective in many photovoltaic applications.
Reference is also made to FIG. 1b, which is a schematic block diagram showing the voltage drop on output protection series diodes of a conventional solar module 20. Furthermore:Vout=Vp−Vds[V]  (2)where Vout is the total voltage produced by the module including voltage drop on series diode 27. This voltage drop may be the reason for module additional power losses.
Practically, two diodes electrically connected in series are used in order to avoid diode breakdown voltage. If a cell in a serial-unit 26 malfunctions for any reason, the power produced by the whole serial-unit 26 is lost. The power produced by the module is computed as follows:
                    Pout        =                              Pp            (                          1              -                              xn                m                            -                                                                    2                    *                    Vds                                    +                                      n                    *                    Vdp                                                                                        Vp                    *                                          (                                              m                        -                        xn                                            )                                                        m                                                      )                    ⁢                                          [          W          ]                                    (        1        )            where    Pout is the total power produced by the module, including power loss on series diodes 27.    Pp is the maximum power that solar-array module 20 produces when all cells 22 12 function,    x is the number of cells 22 in a serial-unit 26,    n is the number of malfunctioning serial-units 26,    Vds is the voltage drop over a diode 27 electrically connected in series with solar-array module 20,    Vdp is the voltage drop over bypass diode 25 electrically connected in parallel with a serial-unit 26, and    Vp is the voltage that solar-array module 20 produces when all cells 22 function.
It should be noted that equation (1) is an approximation and is suitable for x*n≦45.
Reference is now made to FIG. 2—a block diagram, showing a prior art solar-array module 30. Solar-array module 30 includes solar cells 22 electrically connected in series to form serial-units 26. In the example shown in FIG. 2, each serial-unit 26 includes 4 solar cells 22. The serial-units 26 are interconnected in parallel (32), to obtain a desired current producing capacity for solar array module 30. The number of solar cells 22 that form a serial-unit 26 determines the voltage level provided by solar-array module 30. The number of serial-units 26, electrically connected in parallel, determines the current level provided by solar-array module 30, to thereby obtain the predetermined electric power.
For example, a solar module 30 includes 60 solar cells 22, wherein each serial-unit 26 includes 4 solar cells 22 and wherein 15 serial-units 26 are electrically interconnected in parallel. For solar cells 22 that produce 0.5 Volt each, each serial-unit 26 produces 2 Volts.
A power converter 34 is connected at the exit of the array of solar cells 22 of module 30, which power converter 34 converts the input voltage level (2 Volts, in the afore mentioned example) to a significantly higher output voltage level, for example 30 Volts, in the afore mentioned example. Hence, when a solar cell 22 that is a member of a serial-unit 26 is defective, the module loses the power of the whole serial-unit 26.
Therefore, there is a need and it would be advantageous be able to prevent power loses of the whole serial-unit 26 as a result of a malfunction solar cell.
Reference is also made to FIG. 3—a block diagram, showing a prior art solar-array module 40. Solar-array module 40 is similar to solar array module 30, except that each serial-unit 26 includes a single solar cell 22. Solar module 40 is optimal in the sense that when a solar cell 22 malfunctions, the only power loss is the power of the serial-unit 26 containing the malfunctioning solar cell 22.
In an optimal solar module the power loses are very small.
Typically, the voltage level of a conventional solar cell 22 is relatively low (about 0.5V). With this level of input voltage, the input current of power converter 44 for solar module, for example of 250 W, will be very high (more than 500 A), and power converter 44 efficiency is not high enough to provide such current level. Therefor, there is a need and it would be advantageous be able to provide a higher input voltage to power converter 44.
It should be noted that throughout the present disclosure, the invention is described using the text and related drawings. The equations are included only as a possible help to persons skilled in the art, and should not be considered as limiting the invention in any way. Various other equations may be used by persons skilled in the art.
There is a need for and it would be advantageous to have an apparatus, system and method for solar electric power generation, wherein the apparatus facilitates maximization of the power generated by a solar-array module in which module one or more Silicon solar cells malfunction.